Tropical troposphere to stratosphere transport of carbon monoxide and long-lived trace species in the Chemical Lagrangian Model of the Stratosphere (CLaMS)

Abstract

Abstract. Variations in the mixing ratio of trace gases of tropospheric origin entering the stratosphere in the tropics are of interest for assessing both troposphere to stratosphere transport fluxes in the tropics and the impact of these transport fluxes on the composition of the tropical lower stratosphere. Anomaly patterns of carbon monoxide (CO) and long-lived tracers in the lower tropical stratosphere allow conclusions about the rate and the variability of tropical upwelling to be drawn. Here, we present a simplified chemistry scheme for the Chemical Lagrangian Model of the Stratosphere (CLaMS) for the simulation, at comparatively low numerical cost, of CO, ozone, and long-lived trace substances (CH4, N2O, CCl3F (CFC-11), CCl2F2 (CFC-12), and CO2) in the lower tropical stratosphere. For the long-lived trace substances, the boundary conditions at the surface are prescribed based on ground-based measurements in the lowest model level. The boundary condition for CO in the lower troposphere (below about 4 km) is deduced from MOPITT measurements. Due to the lack of a specific representation of mixing and convective uplift in the troposphere in this model version, enhanced CO values, in particular those resulting from convective outflow are underestimated. However, in the tropical tropopause layer and the lower tropical stratosphere, there is relatively good agreement of simulated CO with in situ measurements (with the exception of the TROCCINOX campaign, where CO in the simulation is biased low ≈10–15 ppbv). Further, the model results (and therefore also the ERA-Interim winds, on which the transport in the model is based) are of sufficient quality to describe large scale anomaly patterns of CO in the lower stratosphere. In particular, the zonally averaged tropical CO anomaly patterns (the so called "tape recorder" patterns) simulated by this model version of CLaMS are in good agreement with observations, although the simulations show a too rapid upwelling compared to observations as a consequence of the overestimated vertical velocities in the ERA-Interim reanalysis data set. Moreover, the simulated tropical anomaly patterns of N2O are in good agreement with observations. In the simulations, anomaly patterns of CH4 and CFC-11 were found to be very similar to those of N2O; for all long-lived tracers, positive anomalies are simulated because of the enhanced tropical upwelling in the easterly shear phase of the quasi-biennial oscillation.

Quantifying pollution transport from the Asian monsoon anticyclone into the lower stratosphere

“…Also, for the tropopause-based map, CO distributions from CLaMS and MLS observations agree reliably well in the monsoon region. Significant differences between CLaMS and MLS exist only at midlatitudes (already observed by Pommrich et al, 2014) and above the west Pacific and Maritime continent. Figure 3c shows analogous maps as for CO for the anticyclone air mass fraction.…”

“…We included an air mass origin tracer in the model to diagnose the fraction of air at any location in the stratosphere which has left the Asian monsoon anticyclone during the previous monsoon season (see below). In addition we consider carbon monoxide (CO), with the CO lower boundary in CLaMS derived from Atmospheric Infrared Sounder (AIRS) version 6 satellite measurements following the method described in Pommrich et al (2014), with relevant chemistry for the UTLS region included (Pommrich et al, 2014).…”

“…We quantify air mass transport from the Asian monsoon anticyclone using simulations with the Lagrangian chemistry transport model CLaMS (McKenna et al, 2002;Konopka et al, 2004;Pommrich et al, 2014). CLaMS uses an isentropic vertical coordinate throughout the UTLS, and the model transport is driven with horizontal winds and total diabatic heating rates from European Centre of MediumRange Weather Forecasts (ECMWF) ERA-Interim reanalysis (Dee et al, 2011).…”

Effects of mixing on resolved and unresolved scales on stratospheric age of air

“…2.3). Further details of the model setup used here can be found in Pommrich et al (2014). A particular advantage of CLaMS Lagrangian transport is that the trajectory calculation is non-diffusive per se, and that the strength of diffusion induced by parametrized small-scale mixing may be controlled.…”

“…Crossisentropic vertical velocity has been deduced from the reanalysis forecast total diabatic heating rate (see Pommrich et al, 2014). We carried out a high-resolution reference simulations (CLaMS-ERAI), with the critical Lyapunov exponent chosen as λ c = 1.5 day −1 , resulting in good agreement with observed trace gas distributions as shown in several recent publications (e.g., Pommrich et al, 2014;Ploeger et al, 2015a). Furthermore, for the investigation of model diffusion effects we carried out two low-resolution sensitivity simulations, both driven by ERA-Interim meteorology but with varying the strength of parametrized small-scale mixing (either λ = 1.5 day −1 or λ = 1.0 day −1 ).…”